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Friction stir welding and processing

R.S. Mishraa,*, Z.Y. MabaCenter for Friction Stir Processing, Department of Materials Science and Engineering, University of Missouri,

Rolla, MO 65409, USAb Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Available online 18 August 2005

Abstract

Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy

efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum

alloys and other metallic alloys that arehard to weld by conventional fusion welding. FSWis considered to be the most

significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for

microstructural modification of metallic materials. In this review article, the current state of understanding and

development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible

for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant

microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys,

important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly

outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Friction stir welding; Friction stir processing; Weld; Processing; Microstructure

1. Introduction

The difficulty of making high-strength, fatigue and fracture resistant welds in aerospace aluminumalloys, such as highly alloyed 2XXX and 7XXX series, has long inhibited the wide use of welding for

joining aerospace structures. These aluminum alloys are generally classified as non-weldable because of the poor solidification microstructure and porosity in the fusion zone. Also, the loss in mechanicalproperties as compared to the base material is very significant. These factors make the joining of these

alloys by conventional welding processes unattractive. Some aluminum alloys can be resistance welded,but the surface preparation is expensive, with surface oxide being a major problem.

Friction stir welding (FSW) was invented at The Welding Institute (TWI) of UK in 1991 as asolid-state joining technique, and it was initially applied to aluminum alloys [1,2]. The basic conceptof FSW is remarkably simple. A non-consumable rotating tool with a specially designed pin and

shoulder is inserted into the abutting edges of sheets or plates to be joined and traversed along the lineof joint (Fig. 1). The tool serves two primary functions: (a) heating of workpiece, and (b) movement of material to produce the joint. The heating is accomplished by friction between the tool and theworkpiece and plastic deformation of workpiece. The localized heating softens the material around thepin and combination of tool rotation and translation leads to movement of material from the front of

Materials Science and Engineering R 50 (2005) 1–78

* Corresponding author. Tel.: +1 573 341 6361; fax: +1 573 341 6934.

E-mail address: [email protected] (R.S. Mishra).

0927-796X/$ – see front matter# 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.mser.2005.07.001


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the pin to the back of the pin. As a result of this process a joint is produced in ‘solid state’. Because of various geometrical features of the tool, the material movement around the pin can be quite complex

[3]. During FSW process, the material undergoes intense plastic deformation at elevated temperature,

resulting in generation of fine and equiaxed recrystallized grains [4–7]. The fine microstructure infriction stir welds produces good mechanical properties.

FSW is considered to be the most significant development in metal joining in a decade and is a‘‘green’’ technology due to its energy ef ficiency, environment friendliness, and versatility. Ascompared to the conventional welding methods, FSW consumes considerably less energy. No cover

gas or flux is used, thereby making the process environmentally friendly. The joining does not involveany use of filler metal and therefore any aluminum alloy can be joined without concern for thecompatibility of composition, which is an issue in fusion welding. When desirable, dissimilar

aluminum alloys and composites can be joined with equal ease [8–10]. In contrast to the traditional

friction welding, which is usually performed on small axisymmetric parts that can be rotated and

pushed against each other to form a joint [11], friction stir welding can be applied to various types of

joints like butt joints, lap joints, T butt joints, and fillet joints [12]. The key benefits of FSW aresummarized in Table 1.

Recently friction stir processing (FSP) was developed by Mishra et al. [13,14] as a generic tool for

microstructural modification based on the basic principles of FSW. In this case, a rotating tool isinserted in a monolithic workpiece for localized microstructural modification for specific property

enhancement. For example, high-strain rate superplasticity was obtained in commercial 7075Al alloy

2 R.S. Mishra, Z.Y. Ma / Materials Science and Engineering R 50 (2005) 1 – 78

Fig. 1. Schematic drawing of friction stir welding.

Table 1

Key benefits of friction stir welding

Metallurgical benefits Environmental benefits Energy benefits

Solid phase process

Low distortion of workpiece

Good dimensional stability

and repeatability

No loss of alloying elements

Excellent metallurgical

properties in the joint area

Fine microstructure

Absence of cracking

Replace multiple parts

joined by fasteners

No shielding gas required

No surface cleaning required

Eliminate grinding wastes

Eliminate solvents

required for degreasing

Consumable materials saving,

such as rugs, wire or

any other gases

Improved materials use (e.g., joining different

thickness) allows reduction in weight

Only 2.5% of the energy needed for a

laser weld

Decreased fuel consumption in light weight

aircraft, automotive and ship applications


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by FSP [13–15]. Furthermore, FSP technique has been used to produce surface composite onaluminum substrate [16], homogenization of powder metallurgy aluminum alloy [17], microstructural

modification of metal matrix composites [18] and property enhancement in cast aluminum alloys [19].

FSW/FSP is emerging as a very effective solid-state joining/processing technique. In a relatively

short duration after invention, quite a few successful applications of FSW have been demonstrated

[20–23]. In this paper, the current state of understanding and development of the FSW and FSP arereviewed.

2. Process parameters

FSW/FSP involves complex material movement and plastic deformation. Welding parameters,

tool geometry, and joint design exert significant effect on the material flow pattern and temperature

distribution, thereby influencing the microstructural evolution of material. In this section, a few majorfactors affecting FSW/FSP process, such as tool geometry, welding parameters, joint design are

addressed.

2.1. Tool geometry

Tool geometry is the most influential aspect of process development. The tool geometry plays a

critical role in material flow and in turn governs the traverse rate at which FSW can be conducted. AnFSW tool consists of a shoulder and a pin as shown schematically in Fig. 2. As mentioned earlier, the

tool has two primary functions: (a) localized heating, and (b) material flow. In the initial stage of toolplunge, the heating results primarily from the friction between pin and workpiece. Some additional

heating results from deformation of material. The tool is plunged till the shoulder touches theworkpiece. The friction between the shoulder and workpiece results in the biggest component of

heating. From the heating aspect, the relative size of pin and shoulder is important, and the other design

features are not critical. The shoulder also provides confinement for the heated volume of material.The second function of the tool is to ‘stir’ and ‘move’ the material. The uniformity of microstructureand properties as well as process loads are governed by the tool design. Generally a concave shoulder

and threaded cylindrical pins are used.

With increasing experience and some improvement in understanding of material flow, the toolgeometry has evolved significantly. Complex features have been added to alter material flow, mixingand reduce process loads. For example, WhorlTM and MX TrifluteTM tools developed by TWI are

R.S. Mishra, Z.Y. Ma / Materials Science and Engineering R 50 (2005) 1 – 78 3

Fig. 2. Schematic drawing of the FSW tool.


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shown in Fig. 3. Thomas et al. [24] pointed out that pins for both tools are shaped as a frustum that

displaces less material than a cylindrical tool of the same root diameter. Typically, the WhorlTM reduces

the displaced volume by about 60%, while the MX TrifluteTM reduces the displaced volume by about70%. The design featuresof the WhorlTM andtheMXTrifluteTM are believed to (a) reduce welding force,

(b) enable easier flow of plasticized material, (c) facilitate the downward augering effect,and (d) increasethe interface between the pin and the plasticized material, thereby increasing heat generation. It has been

demonstrated that aluminum plates with a thickness of up to 50 mm can be successfully friction stir

welded in one pass using these two tools. A 75 mm thick 6082Al-T6 FSW weld was made using

WhorlTM tool in two passes, each giving about 38 mm penetration. Thomas et al. [24] suggested that the

major factor determining the superiority of the whorl pins over the conventional cylindrical pins is the

ratio of the swept volume during rotation to the volume of the pin itself, i.e., a ratio of the ‘‘dynamic

volume to the static volume’’ thatis important in providing an adequateflow path. Typically, thisratio forpins with similar root diameters and pin length is 1.1:1 for conventional cylindrical pin, 1.8:1 for the

WhorlTM and 2.6:1 for the MX TrifluteTM pin (when welding 25 mm thick plate).For lap welding, conventional cylindrical threaded pin resulted in excessive thinning of the top

sheet, leading to significantly reduced bend properties [25]. Furthermore, for lap welds, the width of the weld interface and the angle at which the notch meets the edge of the weld is also important for

applications where fatigue is of main concern. Recently, two new pin geometries—Flared-TrifuteTM

with the flute lands being flared out (Fig. 4) and A-skewTM with the pin axis being slightly inclined tothe axis of machine spindle (Fig. 5) were developed for improved quality of lap welding [25–27]. Thedesign features of the Flared-TrifuteTM and the A-skewTM are believed to: (a) increase the ratio

between of the swept volume and static volume of the pin, thereby improving the flow path around and

underneath the pin, (b) widen the welding region due to flared-out flute lands in the Flared-TrifuteTM

pin and the skew action in the A-skewTM pin, (c) provide an improved mixing action for oxide

fragmentation and dispersal at the weld interface, and (d) provide an orbital forging action at the root

of the weld due to the skew action, improving weld quality in this region. Compared to the


Fig. 3. WorlTM

and MX TrifluteTM tools developed by The Welding Institute (TWI), UK (Copyright# 2001, TWI Ltd) (afterThomas et al. [24]).


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conventional threaded pin, Flared-TrifuteTM and A-skewTM pins resulted in: (a) over 100% improve-

ment in welding speed, (b) about 20% reduction in axial force, (c) significantly widened weldingregion (190–195% of the plate thickness for Flared-TrifuteTM and A-skewTM pins, 110% for

conventional threaded pin), and (d) a reduction in upper plate thinning by a factor of >4 [27].Further, Flared-TrifuteTM pin reduced significantly the angle of the notch upturn at the overlappingplate/weld interface, whereas A-skewTM pin produced a slight downturn at the outer regions of the

overlapping plate/weld interface, which are beneficial to improving the properties of the FSW joints

[25,27]. Thomas and Dolby [27] suggested that both Flared-TrifuteTM and A-skewTM pins are suitable

for lap, T, and similar welds where joining interface is vertical to the machine axis.

Further, various shoulder profiles were designed in TWI to suit different materials and conditions(Fig. 6). These shoulder profiles improve the coupling between the tool shoulder and the workpiecesby entrapping plasticized material within special re-entrant features.

Considering the significant effect of tool geometry on the metal flow, fundamental correlationbetween material flow and resultant microstructure of welds varies with each tool. A critical need is to

develop systematic framework for tool design. Computational tools, including finite element analysis


Fig. 4. Flared-TrifluteTM tools developed by The Welding Institute (TWI), UK: (a) neutral flutes, (b) left flutes, and (c) righthand flutes (after Thomas et al. [25]).

Fig. 5. A-SkewTM

tool developed by The Welding Institute (TWI), UK: (a) side view, (b) front view, and (c) swept regionencompassed by skew action (after Thomas et al. [25]).


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(FEA), can be used to visualize the material flow and calculate axial forces. Several companies have

indicated internal R&D efforts in friction stir welding conferences, but no open literature is availableon such efforts and outcome. It is important to realize that generalization of microstructural

development and influence of processing parameters is dif ficult in absence of the tool information.

2.2. Welding parameters

For FSW, two parameters are very important: tool rotation rate (v, rpm) in clockwise or

counterclockwise direction and tool traverse speed (n, mm/min) along the line of joint. The rotation

of tool results in stirring and mixing of material around the rotating pin and the translation of tool

moves the stirred material from the front to the back of the pin and finishes welding process. Highertool rotation rates generate higher temperature because of higher friction heating and result in more

intense stirring and mixing of material as will be discussed later. However, it should be noted that

frictional coupling of tool surface with workpiece is going to govern the heating. So, a monotonic

increase in heating with increasing tool rotation rate is not expected as the coef ficient of friction atinterface will change with increasing tool rotation rate.

In addition to the tool rotation rate and traverse speed, another important process parameter is the

angle of spindle or tool tilt with respect to the workpiece surface. A suitable tilt of the spindle towards

trailing direction ensures that the shoulder of the tool holds the stirred material by threaded pin and

move material ef ficiently from the front to the back of the pin. Further, the insertion depth of pin intothe workpieces (also called target depth) is important for producing sound welds with smooth tool

shoulders. The insertion depth of pin is associated with the pin height. When the insertion depth is too

shallow, the shoulder of tool does not contact the original workpiece surface. Thus, rotating shoulder

cannot move the stirred material ef ficiently from the front to the back of the pin, resulting in generation

of welds with inner channel or surface groove. When the insertion depth is too deep, the shoulder of tool plunges into the workpiece creating excessive flash. In this case, a significantly concave weld is

produced, leading to local thinning of the welded plates. It should be noted that the recent development

of ‘scrolled’ tool shoulder allows FSW with 08 tool tilt. Such tools are particularly preferred for curved joints.

Preheating or cooling can also be important for some specific FSW processes. For materials withhigh melting point such as steel and titanium or high conductivity such as copper, the heat produced by

friction and stirring may be not suf ficient to soften and plasticize the material around the rotating tool.Thus, it is dif ficult to produce continuous defect-free weld. In these cases, preheating or additionalexternal heating source can help the material flow and increase the process window. On the other hand,materials with lower melting point such as aluminum and magnesium, cooling can be used to reduce


Fig. 6. Tool shoulder geometries, viewed from underneath the shoulder (Copyright# 2001, TWI Ltd) (after Thomas et al.[24]).


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extensive growth of recrystallized grains and dissolution of strengthening precipitates in and around

the stirred zone.

2.3. Joint design

The most convenient joint configurations for FSW are butt and lap joints. A simple square butt joint is shown in Fig. 7a. Two plates or sheets with same thickness are placed on a backing plate and

clamped firmly to prevent the abutting joint faces from being forced apart. During the initial plunge of the tool, the forces are fairly large and extra care is required to ensure that plates in butt configuration

do not separate. A rotating tool is plunged into the joint line and traversed along this line when the

shoulder of the tool is in intimate contact with the surface of the plates, producing a weld along

abutting line. On the other hand, for a simple lap joint, two lapped plates or sheets are clamped on a

backing plate. A rotating tool is vertically plunged through the upper plate and into the lower plate and

traversed along desired direction, joining the two plates (Fig. 7d). Many other configurations can be

produced by combination of butt and lap joints. Apart from butt and lap joint configurations, othertypes of joint designs, such as fillet joints (Fig. 7g), are also possible as needed for some engineeringapplications.

It is important to note that no special preparation is needed for FSW of butt and lap joints. Two

clean metal plates can be easily joined together in the form of butt or lap joints without any major

concern about the surface conditions of the plates.

3. Process modeling

FSW/FSP results in intense plastic deformation and temperature increase within and around the

stirred zone. This results in significant microstructural evolution, including grain size, grain boundarycharacter, dissolution and coarsening of precipitates, breakup and redistribution of dispersoids, and

texture. An understanding of mechanical and thermal processes during FSW/FSP is needed for

optimizing process parameters and controlling microstructure and properties of welds. In this section,

the present understanding of mechanical and thermal processes during FSW/FSP is reviewed.

3.1. Metal flow

The material flow during friction stir welding is quite complex depending on the tool geometry,process parameters, and material to be welded. It is of practical importance to understand the material

flow characteristics for optimal tool design and obtain high structural ef ficiency welds. This has led to

R.S. Mishra, Z.Y. Ma / Materials Science and Engineering R 50 (2005) 1–78 7

Fig. 7. Joint configurations for friction stir welding: (a) square butt, (b) edge butt, (c) T butt joint, (d) lap joint, (e) multiplelap joint, (f) T lap joint, and (g) fillet joint.


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numerous investigations on material flow behavior during FSW. A number of approaches, such astracer technique by marker, welding of dissimilar alloys/metals, have been used to visualize material

flow pattern in FSW. In addition, some computational methods including FEA have been also used to

model the material flow.

3.1.1. Experimental observations

The material flow is influenced very significantly by the tool design. Therefore, any general-

ization should be treated carefully. Also, most of the studies do not report tool design and all process

conditions. Therefore, differences among various studies cannot be easily discerned. To develop an

overall pattern, in this review a few studies are specifically summarized and then some general trendsare presented.

3.1.1.1. Tracer technique by marker. One method of tracking the material flow in a friction stir weld

is to use a marker material as a tracer that is different from the material being welded. In the past fewyears, different marker materials, such as aluminum alloy that etch differently from the base metal

[28–30], copper foil [31], small steel shots [32,33], Al–SiCp and Al–W composites [3,34], and

tungsten wire [35], have been used to track the material flow during FSW.Reynolds and coworkers [28–30] investigated the material flow behavior in FSW 2195Al-T8

using a marker insert technique (MIT). In this technique, markers made of 5454Al-H32 were

embedded in the path of the rotating tool as shown in Fig. 8 and their final position after weldingwas revealed by milling off successive slices of 0.25 mm thick from the top surface of the weld,

etching with Keller’s reagent, and metallographic examination. Further, a projection of the marker

positions onto a vertical plane in the welding direction was constructed. These investigations revealed

the following. First, all welds exhibited some common flow patterns. The flow was not symmetric

about the weld centerline. Bulk of the marker material moved to a final position behind its originalposition and only a small amount of the material on the advancing side was moved to a final position infront of its original position. The backward movement of material was limited to one pin diameter

behind its original position. Second, there is a well-defined interface between the advancing andretreating sides, and the material was not really stirred across the interface during the FSW process, at

least not on a macroscopic level. Third, material was pushed downward on the advancing side and

moved toward the top at the retreating side within the pin diameter. This indicates that the ‘‘stirring’’ of material occurred only at the top of the weld where the material transport was directly influenced bythe rotating tool shoulder that moved material from the retreating side around the pin to the advancing


Fig. 8. Schematic drawing of the marker configuration (after Reynolds [29]).


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side. Fourth, the amount of vertical displacement of the retreating side bottom marker was inversely

proportional to theweld pitch (welding speed/rotation rate, i.e. the tool advance per rotation). Fifth, the

material transport across the weld centerline increased with increasing the pin diameter at a constant

tool rotation rate and traverse speed. Based on these observations, Reynolds et al. [29,30] suggested

that the friction stir welding process can be roughly described as an in situ extrusion process wherein

the tool shoulder, the pin, the weld backing plate, and cold base metal outside the weld zone form an

‘‘extrusion chamber’’ which moves relative to the workpiece. They concluded that the extrusion

around the pin combined with the stirring action at the top of the weld created within the pin diameter a

secondary, vertical, circular motion around the longitudinal axis of the weld.

Guerra et al. [31] studied the material flow of FSW 6061Al by means of a faying surface tracerand a pin frozen in place at the end of welding. For this technique, weld was made with a thin

0.1 mm high-purity Cu foil along the faying surface of the weld. After a stable weld had been

established, the pin rotation and specimen translation were manually stopped to produce a pin

frozen into the workpiece. Plan view and transverse metallographic sections were examinedafter etching. Based on the microstructural examinations, Guerra et al. [31] concluded that the

material was moved around the pin in FSW by two processes. First, material on the advancing side

front of a weld entered into a zone that rotates and advances simultaneously with the pin. The

material in this zone was very highly deformed and sloughed off behind the pin in arc shaped

features. This zone exhibited high Vicker’s microhardness of 95. Second, material on the retreatingfront side of the pin extruded between the rotational zone and the parent metal and in the wake of

the weld fills in between material sloughed off from the rotational zone. This zone exhibited lowVicker’s microhardness of 35. Further, they pointed out that material near the top of the weld(approximately the upper one-third) moved under the influence of the shoulder rather than thethreads on the pin.

Colligan [32,33] studied the material flow behavior during FSWof aluminum alloys by means of

steel shot tracer technique and ‘‘stop action’’ technique. For the steel shot tracer technique, a line of small steel balls of 0.38 mm diameter were embedded along welding direction at different positions

within butt joint welds of 6061Al-T6 and 7075Al-T6 plates. After stopping welding, each weld was

subsequently radiographed to reveal the distribution of the tracer material around and behind the pin.

The ‘‘stop action’’ technique involved terminating friction stir welding by suddenly stopping theforward motion of the welding tool and simultaneously retracting the tool at a rate that caused the

welding tool pin to unscrew itself from the weld, leaving the material within the threads of the pin

intact and still attached to the keyhole. By sectioning the keyhole, the flow pattern of material in theregion immediately within the threads of the welding tool was revealed. These investigations revealed

the following important observations. First, the distribution of the tracer steel shots can be divided into

two general categories: chaotical and continuous distribution. In the regions near top surface of the

weld, individual tracer elements were scattered in an erratic way within a relatively broad zone behindthe welding tool pin, i.e., chaotical distribution. The chaotically deposited tracer steel shots had moved

to a greater depth from their original position. In other regions of the weld, the initial continuous line of

steel shots was reorientated and deposited as a roughly continuous line of steel shot behind the pin, i.e.,

continuous distribution. However, the tracer steel shots were found to be little closer to the upper

surface of the weld. Second, in the leading side of the keyhole, the thread form gradually developed

from curls of aluminum. The continuous downward motion of the thread relative to the forward

advance of the pin caused the material captured inside the thread space to be deposited behind the pin.

Based on these observations, Colligan [32,33] concluded that not all the material in the tool path was

actually stirred and rather a large amount of the material was simply extruded around the retreating

side of the welding tool pin and deposited behind. However, it should be pointed out that if the marker



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material has different flow strength and density, it can create uncertainty about the accuracy of theconclusions.

London et al. [34] investigated material flow in FSW of 7050Al-T7451 monitored with 6061Al–

30 vol.% SiCp and Al–20 vol.% W composite markers. The markers with a cross-section of 0.79 mm 0.51 mm were placed at the center on the midplane of the workpiece (MC) and at theadvancing side on the midplane (MA). In each FSW experiment, the forward progress of the tool was

stopped while in the process of spreading the marker. The distribution of marker material was

examined by metallography and X-ray. Based on experimental observations, London et al. [34]

suggested that the flow of the marker in the FSW zone goes through the following sequence of events.First, material ahead of the pin is significantly uplifted because of the 38 tilt of the tool, which creates a‘‘plowing action’’ of the metal ahead of the weld. Second, following this uplift, the marker is shearedaround the periphery of the pin while at the same time it is being pushed downward in the plate because

of the action of the threads. Third, marker material is dropped off behind the pin in ‘‘streaks’’ which

correspond to the geometry of the threads and specific weld parameters used to create these welds.Furthermore, London et al. [34] showed that the amount of material deformation in the FSW weld

depends on the locations relative to the pin. Markers on the advancing side of the weld are distributed

over a much wider region in the wake of the weld than markers that begin at the weld centerline.

3.1.1.2. Flow visualization by FSW of dissimilar materials. In addition to the tracer technique,

several studies have used friction stir welding of dissimilar metals for visualizing the complex flow

phenomenon. Midling [35] investigated the influence of the welding speed on the material flow inwelds of dissimilar aluminum alloys. He was the first to report on interface shapes using images of themicrostructure. However, information on flow visualization was limited to the interface betweendissimilar alloys.

Ouyang and Kovacevic [36] examined the material flow behavior in friction stir butting welding

of 2024Al to 6061Al plates of 12.7 mm thick. Three different regions were revealed in the welded

zone. The first was the mechanically mixed region characterized by the relatively uniformly dispersedparticles of different alloy constituents. The second was the stirring-induced plastic flow regionconsisting of alternative vortex-like lamellae of the two aluminum alloys. The third was the unmixed

region consisting of fine equiaxed grains of the 6061Al alloy. They reported that in the welds thecontact between different layers is intimate, but the mixing is far from complete. However, the bonding

between the two aluminum alloys was complete. Further, they attributed the vortex-like structure and

alternative lamellae to the stirring action of the threaded tool, in situ extrusion, and traverse motion

along the welding direction.

Murr and co-workers [8,10,37,38] investigated the solid-state flow visualization in friction stirbutt welding of 2024Al to 6061Al and copper to 6061Al. The material flow was described as a

chaotic–dynamic intercalation microstructures consisting of vortex-like and swirl features. Theyfurther suggested that the complex mixing and intercalation of dissimilar metals in FSW is essentially

the same as the microstructures characteristic of mechanically alloyed systems. On the other hand, a

recent investigation on friction stir lap welding of 2195Al to 6061Al revealed that there is large vertical

movement of material within the rotational zone caused by the wash and backwash of the threads [31].

Guerra et al. [31] have stated that material entering this zone followed an unwound helical trajectory

formed by the rotational motion, the vertical flow, and the translational motion of the pin.

3.1.1.3. Microstructural observations. The idea that the FSW is likened to an extrusion process is

also supported by Krishnan [39]. Krishnan [39] investigated the formation of onion rings in friction stir

welds of 6061Al and 7075Al alloys by using different FSW parameters. Onion rings found in the



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welded zone is a direct evidence of characteristic material transport phenomena occurring during

FSW. It was suggested that the friction stir welding process can be thought to be simply extruding one

layer of semicylinder in one rotation of the tool and a cross-sectional slice through such a set of

semicylinder results in the familiar onion ring structure. On the other hand, Biallas et al. [40] suggested

that the formation of onion rings was attributed to the reflection of material flow approximately at theimaginary walls of the groove that would be formed in the case of regular milling of the metal. The

induced circular movement leads to circles that decrease in radii and form the tube system. In this case,

it is believed that there should be thorough mixing of material in the nugget region. Although

microstructural examinations revealed an abrupt variation in grain size and/or precipitate density at

these rings [41,42], it is noted that the understanding of formation of onion rings is far from complete

and an insight into the mechanism of onion ring formation would shed light on the overall material

flow occurring during FSW.Recently, Ma et al. [43] conducted a study on microstructural modification of cast A356 via

friction stir processing. As-cast A356 plates were subjected to friction stir processing by usingdifferent tool geometries and FSP parameters. Fig. 9 shows the optical micrographs of as-cast A356

and FSP sample prepared using a standard threaded pin and tool rotation rate of 900 rpm and traverse

speed of 203 mm/min. The as-cast A356 was characterized by coarse acicular Si particles with an

aspect ratio of up to 25, coarse primary aluminum dendrites with an average size of 100 mm, andporosity of 50 mm diameter (Fig. 9a). The acicular Si particles were preferentially distributed alongthe boundaries of the primary aluminum dendrites, i.e., the distribution of Si particles in the as-cast

A356 was not uniform. FSP resulted in a significant breakup of acicular Si particles and aluminumdendrites. A uniform redistribution of the broken Si particles in the aluminum matrix was also

produced. After FSP, the average aspect ratio of Si was reduced to 2.0. Further, FSP also eliminatedthe porosity in the as-cast A356. Clearly, the material within the processed zone of the FSP A356

experienced intense stirring and mixing, thereby resulting in breakup of the coarse acicular Si particles

and dendrite structure and homogeneous distribution of the Si particles throughout the aluminum

matrix. Previous investigations have indicated that the extrusion at high temperature does not reduce

the high-aspect-ratio reinforcements to nearly equiaxed particles [44,45]. Besides, as-extruded metal

matrix composites are usually characterized by alternative particle-rich bands and particle-free bands

[45,46]. Therefore, in the case of FSP A356 under the experimental conditions used, the material flowwithin the nugget zone cannot be considered as a simple extrusion process.


Fig. 9. Optical micrographs showing the microstructure of as-cast and FSP A356 (standard threaded pin, 900 rpm and203 mm/min) [43].


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3.1.2. Material fl ow modeling

Apart from experimental approaches, a number of studies have been carried out to model the

materials flow during FSWusing different computational codes [47–53], mathematical modeling tools

[54,55], simple geometrical model [56], and metalworking model [57]. These attempts were aimed at

understanding the basic physics of the material flow occurring during FSW.Xu et al. [47] developed two finite element models, the slipping interface model and the frictional

contact model, to simulate the FSW process. The simulation predictions of the material flow pattern

based on these finite element models compare qualitatively well with an experimentally measuredpattern by means of marker insert technique [29,30].

Colegrove and Shercliff [49] modeled the metal flow around profiled FSW tools using a two-dimensional Computational Fluid Dynamics (CFD) code, Fluent. A ‘slip’ model was developed,where the interface conditions were governed by the local shear stresses. The two-dimensional

modeling resulted in the following important findings. First, flow behavior obtained by the slip model

is significantly different from that obtained by the common assumption of material stick. The slipmodel revealed significant differences in flow with different tool shapes, which is not evident with theconventional stick model. Second, the deformation region for the slip model is much smaller on the

advancing side than retreating side. Third, the material in the path of the pin is swept round the

retreating side of the tool. This characteristic of the model is supported by flow visualization

experiments by London et al. [3,34] and Guerra et al. [31]. Fourth, the streamlines show a bulge

behind the tool, and the dragging of material behind the pin on the advancing side. This correlated well

with previous embedded marker experiments by Reynolds and co-workers [29,30].

Smith et al. [50] and Bendzsak and Smith [51] developed a thermo-mechanical flow model(STIR-3D). The principles of fluid mechanics were applied in this model. It assumes viscous heatdissipation as opposed to frictional heating. This model uses tool geometry, alloy type, tool rotation

speed, tool position and travel speed as inputs and predicts the material flow profiles, process loads,

and thermal profiles. It was indicated that three quite distinct flow regimes were formed below the toolshoulder, namely, (a) a region of rotation immediately below the shoulder where flow occurred in thedirection of tool rotation, (b) a region where material is extruded past the rotating tool and this

occurred towards the base of the pin, and (c) a region of transition in between regions (a) and (b) where

the flow had chaotic behavior.Askari et al. [52] adapted a CTH code [58] that is a three-dimensional code capable of solving

time-dependent equations of continuum mechanics and thermodynamics. This model predicts

important fields like strain, strain rate and temperature distribution. The validity of the model wasverified by previous marker insert technique [3,34]. Goetz and Jata [53] used a two-dimensional FEMcode, DEFORM [59], to simulate material flow in FSW of 1100Al and Ti–6Al–4V alloys. Non-isothermal simulation showed that highly localized metal flow is likely to occur during FSW. The

movement of tracking points in these simulations shows metal flow around the tool from one side tothe other, creating a weld. The simulations predict strain rates of 2–12 s1 and strains of 2–5 in the

zone of localized flow.Stewart et al. [54] proposed two models for FSW process, mixed zone model and single slip

surface model. Mixed zone model assumes that the metal in the plastic zone flows in a vortex system atan angular velocity of the tool at the tool–metal interface and the angular velocity drops to zero at theedge of the plastic zone. In the single slip surface model, the principal rotational slip takes place at a

contracted slip surface outside the tool–workpiece interface. It was demonstrated that using a limitedregion of slip, predictions of the thermal field, the force and the weld region shape were in agreementwith experimental measurement. Nunes [55] developed a detailed mathematical model of wiping flowtransfer. This model is found to have the in-built capability to describe the tracer experiments.



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Recently, Arbegast [57] suggested that the resultant microstructure and metal flow features of afriction stir weld closely resemble hot worked microstructure of typical aluminum extrusion and

forging. Therefore, the FSW process can be modeled as a metalworking process in terms of five

conventional metal working zones: (a) preheat, (b) initial deformation, (c) extrusion, (d) forging, and

(e) post heat/cool down (Fig. 10). In the preheat zone ahead of the pin, temperature rises due to the

frictional heating of the rotating tool and adiabatic heating because of the deformation of material. The

thermal properties of material and the traverse speed of the tool govern the extent and heating rate of

this zone. As the tool moves forward, an initial deformation zone forms when material is heated to

above a critical temperature and the magnitude of stress exceeds the critical flow stress of the material,resulting in material flow. The material in this zone is forced both upwards into the shoulder zone anddownwards into the extrusion zone, as shown in Fig. 10. A small amount of material is captured in the

swirl zone beneath the pin tip where a vortex flow pattern exists. In the extrusion zone with a finitewidth, material flows around the pin from the front to the rear. A critical isotherm on each side of the

tool defines the width of the extrusion zone where the magnitudes of stress and temperature areinsuf ficient to allow metal flow. Following the extrusion zone is the forging zone where the materialfrom the front of the tool is forced into the cavity left by the forward moving pin under hydrostatic

pressure conditions. The shoulder of the tool helps to constrain material in this cavity and also applies

a downward forging force. Material from shoulder zone is dragged across the joint from the retreating

side toward the advancing side. Behind the forging zone is the post heat/cool zone where the material

cools under either passive or forced cooling conditions. Arbegast [57] developed a simple approach to

metal flow modeling of the extrusion zone using mass balance considerations that reveals arelationship between tool geometry, operating parameters, and flow stress of the materials being joined. It was indicated that the calculated temperature, width of the extrusion zone, strain rate, and

extrusion pressure are consistent with experimental observations.

In summary, the material flow during FSW is complicated and the understanding of deformation

process is limited. It is important to point out that there are many factors that can influence the materialflow during FSW. These factors include tool geometry (pin and shoulder design, relative dimensions of pin and shoulder), welding parameters (tool rotation rate and direction, i.e., clockwise or counter-

clockwise, traverse speed, plunge depth, spindle angle), material types, workpiece temperature, etc. It

is very likely that the material flow within the nugget during FSW consists of several independentdeformation processes.


Fig. 10. (a) Metal flow patterns and (b) metallurgical processing zones developed during friction stir welding (after Arbegast[57]).


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3.2. Temperature distribution

FSW results in intense plastic deformation around rotating tool and friction between tool and

workpieces. Both these factors contribute to the temperature increase within and around the stirred

zone. Since the temperature distribution within and around the stirred zone directly influences themicrostructure of the welds, such as grain size, grain boundary character, coarsening and dissolution of

precipitates, and resultant mechanical properties of the welds, it is important to obtain information

about temperature distribution during FSW. However, temperature measurements within the stirred

zone are very dif ficult due to the intense plastic deformation produced by the rotation and translationof tool. Therefore, the maximum temperatures within the stirred zone during FSW have been either

estimated from the microstructure of the weld [4,5,60] or recorded by embedding thermocouple in the

regions adjacent to the rotating pin [41,61–63].An investigation of microstructural evolution in 7075Al-T651 during FSW by Rhodes et al. [4]

showed dissolution of larger precipitates and reprecipitation in the weld center. Therefore, theyconcluded that maximum process temperatures are between about 400 and 480 8C in an FSW 7075Al-

T651. On the hand, Murr and co-workers [5,60] indicated that some of the precipitates were not

dissolved during welding and suggested that the temperature rises to roughly 400 8C in an FSW

6061Al. Recently, Sato et al. [61] studied the microstructural evolution of 6063Al during FSW using

transmission electron microscopy (TEM) and compared it with that of simulated weld thermal cycles.

They reported that the precipitates within the weld region (0–8.5 mm from weld center) were

completely dissolved into aluminum matrix. By comparing with the microstructures of simulated

weld thermal cycles at different peak temperatures, they concluded that the regions 0–8.5, 10, 12.5,and 15 mm away from the friction stir weld center were heated to temperatures higher than 402, 353,

302 8C and lower than 201 8C, respectively.

Recently, Mahoney et al. [41] conducted friction stir welding of 6.35 mm thick 7075Al-T651

plate and measured the temperature distribution around the stirred zone both as a function of distance

from the stirred zone and through the thickness of the sheet. Fig. 11 shows the peak temperature

distribution adjacent to the stirred zone. Fig. 11 reveals three important observations. First, maximum

temperature was recorded at the locations close to the stirred zone, i.e., the edge of the stirred zone, and

the temperature decreased with increasing distance from the stirred zone. Second, the temperature at


Fig. 11. Peak temperature distribution adjacent to a friction stir weld in 7075Al-T651. The line on the right side of figureshows the nugget boundary (after Mahoney et al. [41]).


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the edge of the stirred zone increased from the bottom surface of the plate to the top surface. Third, a

maximum temperature of 475 8C was recorded near the corner between the edge of the stirred zone

and the top surface. This temperature is believed to exceed the solution temperature for the hardening

precipitates in 7075Al-T651 [64–66]. Based on these results the temperature within the stirred zone islikely to be above 475 8C. However, the maximum temperature within the stirred zone should be lower

than the melting point of 7075Al because no evidence of material melting was observed in the weld

[4,41].

More recently, an attempt was made by Tang et al. [62] to measure the heat input and temperature

distribution within friction stir weld by embedding thermocouples in the region to be welded. 6061Al-

T6 aluminum plates with a thickness of 6.4 mm were used. They embedded thermocouples in a series

of small holes of 0.92 mm diameter at different distances from weld seam drilled into the back surface

of the workpiece. Three depths of holes (1.59, 3.18, and 4.76 mm) were used to measure the

temperature field at one quarter, one half, and three quarter of the plate thickness. They reported that

the thermocouple at the weld center was not destroyed by the pin during welding but did changeposition slightly due to plastic flow of material ahead of the pin [62]. Fig. 12 shows the variation of thepeak temperature with the distance from the weld centerline for various depths below the top surface.

Three important observations can be made from this plot. First, maximum peak temperature was

recorded at the weld center and with increasing distance from the weld centerline, the peak

temperature decreased. At a tool rotation rate of 400 rpm and a traverse speed of 122 mm/min, a

peak temperature of 450 8C was observed at the weld center one quarter from top surface. Second,there is a nearly isothermal region 4 mm from the weld centerline. Third, the peak temperaturegradient in the thickness direction of the welded joint is very small within the stirred zone and between

25 and 40 8C in the region away from the stirred zone. This indicates that the temperature distribution

within the stirred zone is relatively uniform. Tang et al. [62] further investigated the effect of weld

pressure and tool rotation rate on the temperature field of the weld zone. It was reported that increasing

both tool rotation rate and weld pressure resulted in an increase in the weld temperature. Fig. 13 shows

the effect of tool rotation rate on the peak temperature as a function of distance from the weld

centerline. Clearly, within the weld zone the peak temperature increased by almost 40 8C with

increasing tool rotation rate from 300 to 650 rpm, whereas it only increased by 20 8C when the tool

rotation rate increased from 650 to 1000 rpm, i.e., the rate of temperature increase is lower at higher


Fig. 12. Effect of depth on peak temperature as a function of distance from weld centerline for a 6061Al-T6 FSW weld madeat 400 rpm and 120 mm/min traverse speed (after Tang et al. [62]).


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tool rotation rates. Furthermore, Tang et al. [62] studied the effect of shoulder on the temperature fieldby using two tools with and without pin. The shoulder dominated the heat generation during FSW

(Fig. 14). This was attributed to the fact that the contact area and vertical pressure between the

shoulder and workpiece is much larger than those between the pin and workpiece, and the shoulder has

higher linear velocity than the pin with smaller radius [62]. Additionally, Tang et al. [62] showed that

the thermocouples placed at equal distances from the weld seam but on opposite sides of the weld

showed no significant differences in the temperature.Similarly, Kwon et al. [63], Sato et al. [67], and Hashimoto et al. [68] also measured the

temperature rise in the weld zone by embedding thermocouples in the regions adjacent to the rotating

pin. Kwon et al. [63] reported that in FSW 1050Al, the peak temperature in the FSP zone increased

linearly from 190 to 310 8C with increasing tool rotation rate from 560 to 1840 rpm at a constant tool

traverse speed of 155 mm/min. An investigation by Sato et al. [67] indicated that in FSW 6063Al, the

peak temperature of FSW thermal cycle increased sharply with increasing tool rotation rate from 800


Fig. 13. Effect of tool rotation rate on peak temperature as a function of distance from weld centerline for a 6061Al-T6 FSWweld made at 120 mm/min traverse speed (after Tang et al. [62]).

Fig. 14. Variation of peak temperature with distance from weld centerline for a 6061Al-T6 FSWweld made with and withoutpin (400 rpm and 120 mm/min traverse speed) (after Tang et al. [62]).


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to 2000 rpm at a constant tool traverse speed of 360 mm/min, and above 2000 rpm, however, it rose

gradually with increasing rotation rate from 2000 to 3600 rpm. Peak temperature of >500 8C wasrecorded at a high tool rotation rate of 3600 rpm. Hashimoto et al. [68] reported that the peak

temperature in the weld zone increases with increasing the ratio of tool rotation rate/traverse speed for

FSW of 2024Al-T6, 5083Al-O and 7075Al-T6 (Fig. 15). A peak temperature >550 8C was observedin FSW 5083Al-O at a high ratio of tool rotation rate/traverse speed.

In a recent investigation, a numerical three-dimensional heat flow model for friction stir welding

of age hardenable aluminum alloy has been developed by Frigaad et al. [69], based on the method of

finite differences. The average heat input per unit area and time according to their model is [69]:

q0 ¼ 43p2mPv R3; (1)where q0 is the net power (W), m the friction coef ficient, P the pressure (Pa), v the tool rotational speed(rot/s) and R is the tool radius (m). Frigaad et al. [69] suggested that the tool rotation rate and shoulder

radius are the main process variables in FSW, and the pressure P cannot exceed the actual flow stress of the material at the operating temperature if a sound weld without depressions is to be obtained. The

process model was compared with in situ thermocouple measurements in and around the FSW zone.

FSW of 6082Al-T6 and 7108Al-T79 was performed at constant tool rotation rate of 1500 rpm and a

constant welding force of 7000 N, at three welding speeds of 300, 480, and 720 mm/min. They

revealed three important observations. First, peak temperature of above 500 8C was recorded in theFSW zone. Second, peak temperature decreased with increasing traverse speeds from 300 to 720 mm/

min. Third, the three-dimensional numerical heat flow model yields a temperature–time pattern that isconsistent with that observed experimentally. Similarly, three-dimensional thermal model based on

finite element analysis developed by Chao and Qi [70] and Khandkar and Khan [71] also showedreasonably good match between the simulated temperature profiles and experimental data for both butt

and overlap FSW processes.The effect of FSW parameters on temperature was further examined by Arbegast and Hartley

[72]. They reported that for a given tool geometry and depth of penetration, the maximum temperature

was observed to be a strong function of the rotation rate (v, rpm) while the rate of heating was a strong

function of the traverse speed (n, rpm). It was also noted that there was a slightly higher temperature on

the advancing side of the joint where the tangential velocity vector direction was same as the forward


Fig. 15. Effect of tool rotation rate/traverse speed (v / n) ratio on peak temperature of FSW 2024Al-T6, 5083Al-O, and7075Al-T6 (after Hashimoto et al. [68]).


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velocity vector. They measured the average maximum temperature on 6.35 mm aluminum plates as a

function of the pseudo-‘‘heat index wð

w¼

v2=nÞ

’’. It was demonstrated that for several aluminumalloys a general relationship between maximum welding temperature (T , 8C) and FSW parameters (v,

n) can be explained by

T

T m¼ K v

2

n 104 a

; (2)

where the exponent a was reported to range from 0.04 to 0.06, the constant K is between 0.65 and 0.75,

and T m (8C) is the melting point of the alloy. The maximum temperature observed during FSW of

various aluminum alloys is found to be between 0.6T m and 0.9T m, which is within the hot working

temperature range for those aluminum alloys. Furthermore, the temperature range is generally within

the solution heat-treatment temperature range of precipitation-strengthened aluminum alloys.

Recently, Schmidt et al. [73] have developed an analytical model for the heat generation in FSW.

The important difference between this model and the previous models is the choice of sticking and

sliding contact conditions. The expressions for total heat generation for sticking, sliding, and partial

sliding/sticking conditions, respectively, are

Qtotal;sticking ¼ 23

ps yield ffiffiffi

3p vðð R3shoulder R3probeÞð1 þ tan aÞ þ R3probe þ 3 R2probe H probeÞ; (3a)

Qtotal;sliding ¼ 23

pm pvðð R3shoulder R3probeÞð1 þ tan aÞ þ R3probe þ 3 R2probe H probeÞ; (3b)

Qtotal ¼ 23

p ds yield ffiffiffi

3p þ 1 dð Þm p

vðð R3shoulder R3probeÞð1 þ tan aÞ þ R3probe

þ 3 R2probe H probeÞ; (3c)where Q is the total heat generation (W), s yield the yield strength (Pa), v the tool angular rotation

rate (rad/s), Rshoulder the tool shoulder radius (m), Rprobe the tool probe radius (m), a the tool

shoulder cone angle (8), H probe the tool probe height (m), p the contact pressure (Pa), and d is the

contact state variable. Schmidt et al. [73] verified the model using 2024Al-T3 alloy. They noted thatthe analytical heat generation estimate correlates with the experimental heat generation. The

experimental heat generation was not proportional to the experimental plunge force. Based on this

they suggested that sticking condition must be present at the tool/matrix interface. It should be

noted, however, that the experiments were only performed at a rotational rate of 400 rpm and awelding speed of 120 mm/min.

In summary, many factors influence the thermal profiles during FSW. From numerous experi-mental investigations and process modeling, we conclude the following. First, maximum temperature

rise within the weld zone is below the melting point of aluminum. Second, tool shoulder dominates

heat generation during FSW. Third, maximum temperature increases with increasing tool rotation rate

at a constant tool traverse speed and decreases with increasing traverse speed at a constant tool rotation

rate. Furthermore, maximum temperature during FSW increases with increasing the ratio of tool

rotation rate/traverse speed. Fourth, maximum temperature rise occurs at the top surface of weld zone.

Various theoretical or empirical models proposed so far present different pseudo-heat index. The

experimental verification of these models is very limited and attempts to correlate various data sets



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with models for this review did not show any general trend. The overall picture includes frictional

heating and adiabatic heating. The frictional heating depends on the surface velocity and frictional

coupling (coef ficient of friction). Therefore, the temperature generation should increase from center of the tool shoulder to the edge of the tool shoulder. The pin should also provide some frictional heating

and this aspect has been captured in the model of Schmidt et al. [73]. In addition, the adiabatic heating

is likely to be maximum at the pin and tool shoulder surface and decrease away from the interface.

Currently, the theoretical models do not integrate all these contributions. Recently, Sharma and Mishra

[74] have observed that the nugget area changes with pseudo-heat index ( Fig. 16). The results indicate

that the frictional condition change from ‘stick ’ at lower tool rotation rates to ‘stick/slip’ at higher tool

rotation rates. The implications are very important and needs to be captured in theoretical and

computational modeling of heat generation.

4. Microstructural evolution

The contribution of intense plastic deformation and high-temperature exposure within the stirred

zone during FSW/FSP results in recrystallization and development of texture within the stirred zone

[7,8,10,15,41,62,63,75–91] and precipitate dissolution and coarsening within and around the stirredzone [8,10,41,62,63]. Based on microstructural characterization of grains and precipitates, three

distinct zones, stirred (nugget) zone, thermo-mechanically affected zone (TMAZ), and heat-affected

zone (HAZ), have been identified as shown in Fig. 17. The microstructural changes in various zones


Fig. 16. Variation of nugget cross-section area with pseudo-heat index [74].

Fig. 17. A typical macrograph showing various microstructural zones in FSP 7075Al-T651 (standard threaded pin, 400 rpmand 51 mm/min).


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have significant effect on postweld mechanical properties. Therefore, the microstructural evolutionduring FSW/FSP has been studied by a number of investigators.

4.1. Nugget zone

Intense plastic deformation and frictional heating during FSW/FSP result in generation of a

recrystallized fine-grained microstructure within stirred zone. This region is usually referred to asnugget zone (or weld nugget) or dynamically recrystallized zone (DXZ). Under some FSW/FSP

conditions, onion ring structure was observed in the nugget zone ( Figs. 17 and 18b). In the interior of

the recrystallized grains, usually there is low dislocation density [4,5]. However, some investigators

reported that the small recrystallized grains of the nugget zone contain high density of sub-boundaries

[61], subgrains [75], and dislocations [92]. The interface between the recrystallized nugget zone and

the parent metal is relatively diffuse on the retreating side of the tool, but quite sharp on the advancing

side of the tool [93].

4.1.1. Shape of nugget zone

Depending on processing parameter, tool geometry, temperature of workpiece, and thermal

conductivity of the material, various shapes of nugget zone have been observed. Basically, nugget zone

can be classified into two types, basin-shaped nugget that widens near the upper surface and ellipticalnugget. Sato et al. [61] reported the formation of basin-shaped nugget on friction stir welding of

6063Al-T5 plate. They suggested that the upper surface experiences extreme deformation and

frictional heating by contact with a cylindrical-tool shoulder during FSW, thereby resulting in

generation of basin-shaped nugget zone. On the other hand, Rhodes et al. [4] and Mahoney et al.

[41] reported elliptical nugget zone in the weld of 7075Al-T651.

Recently, an investigation was conducted on the effect of FSP parameter on the microstructureand properties of cast A356 [94]. The results indicated that lower tool rotation rate of 300–500 rpmresulted in generation of basin-shaped nugget zone, whereas elliptical nugget zone was observed by

FSP at higher tool rotation of >700 rpm (Fig. 18). This indicates that with same tool geometry,different nugget shapes can be produced by changing processing parameters.

Reynolds [29] investigated the relationship between nugget size and pin size. It was reported that

the nugget zone was slightly larger than the pin diameter, except at the bottom of the weld where the

pin tapered to a hemispherical termination (Fig. 19). Further, it was revealed that as the pin diameter

increases, the nugget acquired a more rounded shape with a maximum diameter in the middle of the

weld.

4.1.2. Grain size

It is well accepted that the dynamic recrystallization during FSW/FSP results in generation of fineand equiaxed grains in the nugget zone [7,8,10,15,41,62,63,75–91]. FSW/FSP parameters, tool

geometry, composition of workpiece, temperature of the workpiece, vertical pressure, and active

cooling exert significant influence on the size of the recrystallized grains in the FSW/FSP materials.


Fig. 18. Effect of processing parameter on nugget shape in FSP A356: (a) 300 rpm, 51 mm/min and (b) 900 rpm, 203 mm/ min (standard threaded pin) [94].


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Tables 2 and 3 give a summary of the grain size values for various aluminum alloys under different

FSW/FSP conditions. The tool geometry was not identified in a number of studies. While the typicalrecrystallized grain size in the FSW/FSP aluminum alloys is in the micron range (Table 2), ultrafine-grained (UFG) microstructures (average grain size


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Benavides et al. [7] investigated the effect of the workpiece temperature on the grain size of FSW

2024Al. They [7] reported that decreasing the starting temperature of workpiece from 30 to

30 8C

with liquid nitrogen cooling resulted in a decrease in the peak temperature from 330 to 140 8C at alocation 10 mm away from the weld centerline, thereby leading to a reduction in the grain size from 10

to 0.8 mm in FSW 2024Al. Following the same approach, Su et al. [95] prepared bulk nanostructured7075Al with an average grain size of 100 nm via FSP, using a mixture of water, methanol and dry icefor cooling the plate rapidly behind the tool. On the other hand, Kwon et al. [63,90,91] adopted a cone-

shaped pin with a sharpened tip to reduce the amount of frictional heat generated during FSP of

1050Al. A peak temperature of only 190 8C was recorded in the FSP zone at a tool rotation rate of

560 rpm and a traverse speed of 155 mm/min, which resulted in grain size of 0.5 mm. Similarly, Charit

and Mishra [96] reported that a grain size of 0.68 mm was produced, by using a small diameter toolwith normal threaded pin, in FSP of cast Al–Zn–Mg–Sc at a tool rotation rate of 400 rpm and a traversespeed of 25.4 mm/min. These observations are consistent with the general principles for recrystalliza-

tion [97] where the recrystallized grain size decreases with decreasing annealing temperature.

More recently, Li et al. [10], Ma et al. [15], Sato et al. [67], and Kwon et al. [63,90,91] studied the

influence of processing parameter on the microstructure of FSW/FSP aluminum alloys. It was notedthat the recrystallized grain size can be reduced by decreasing the tool rotation rate at a constant tool

traverse speed [10,63,67,90,91] or decreasing the ratio of tool rotation rate/traverse speed [15]. For

example, Kwon et al. [63,90,91] reported that FSP resulted in generation of the grain size of 0.5, 1–2,and 3–4 mm in 1050Al at tool rotation rate of 560, 980, 1840 rpm, respectively, at a constant traversespeed of 155 mm/min. Similarly, Sato et al. [67] reported the grain size of 5.9, 9.2, and 17.8 mm in

FSW 6063Al at tool rotation rate of 800, 1220, 2450 rpm, respectively, at a constant traverse speed of


Fig. 20. Effect of FSP parameters on nugget grain size in FSP 7075Al-T7651 at processing parameter of: (a) 350 rpm,152 mm/min and (b) 400 rpm, 102 mm/min [15].

Table 3

A summary of ultrafine-grained microstructures produced via FSW/FSP in aluminum alloys

Material Plate

thickness

(mm)

Tool geometry Special cooling Rotation

rate (rpm)

Traverse

speed

(mm/min)

Grain

size

(mm)

References

2024Al-T4 6.5 Threaded, cylindrical Liquid nitrogen 650 60 0.5–0.8 [7]

1050Al 5.0 Conical pin without thread N/A 560 155 0.5 [63,90,91]

7075Al 2 N/R Water, methanol,

dry ice

1000 120 0.1 [95]

Cast Al–Zn–Mg–Sc 6.7 Threaded, cylindrical N/A 400 25.4 0.68 [96]


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360 mm/min. Fig. 20 shows the optical micrographs of FSP 7075Al-T651 processed by using two

different processing parameter combinations. Decreasing the ratio of tool rotation rate/traverse

speed from 400 rpm/102 mm/min to 350 rpm/152 mm/min resulted in a decrease in the recrys-

tallized grain size from 7.5 to 3.8 mm. FSW/FSP at higher tool rotation rate or higher ratio of toolrotation rate/traverse speed results in an increase in both degree of deformation and peak

temperature of thermal cycle. The increase in the degree of deformation during FSW/FSP results

in a reduction in the recrystallized grain size according to the general principles for recrystallization

[97]. On the other hand, the increase in peak temperature of FSW/FSP thermal cycle leads to

generation of coarse recrystallized grains, and also results in remarkable grain growth. A recent

investigation on FSP 7050Al has revealed that the initial size of newly recrystallized grains is on the

order of 25–100 nm [98]. When heated for 1–4 min at 350–450 8C, these grains grow to 2–5 mm, asize equivalent to that found in FSP aluminum alloys [98]. Therefore, the variation of recrystallized

grain size with tool rotation rate or traverse speed in FSW/FSP aluminum alloys depends on which

factor is dominant. The investigations on FSP 1050Al and 7075Al-T651 appear to indicate that thepeak temperature of FSW/FSP thermal cycle is the dominant factor in determining the recrys-

tallized grain size. Thus, the recrystallized grain size in the FSW/FSP aluminum alloys generally

increases with increasing the tool rotation rate or the ratio of tool rotation rate/traverse speed.

Fig. 21 shows the variation of grain size with pseudo-heat index in 2024Al and 7075Al [99]. It

shows that there is an optimum combination of tool rotation rate and traverse speed for generating

the finest grain size in a specific aluminum alloy with same tool geometry and temperature of the

workpiece.

The grain size within the weld zone tends to increase near the top of the weld zone and it decreases

with distance on either side of the weld-zone centerline, and this corresponds roughly to temperature

variation within the weld zone [8,10,41]. For example, Mahoney et al. [100] reported a variation in

grain size from the bottom to the top as well as from the advancing to the retreating side in a 6.35 mm-

thick FSP 7050Al. Fig. 22 shows the distribution of the grain sizes in different locations of the nugget

zone of FSP 7050Al [100]. The average grain size ranges from 3.2 mm at the bottom to 5.3 mm at thetop and 3.5 mm from the retreating side to 5.1 mm on the advancing side. Similarly, in a 25.4 mm thick plate of FSW 2519Al, it was found that the average grain sizes were 12, 8 and 2 mm, respectively, in


Fig. 21. Variation of grain size with pseudo-heat index [99]. Note that the grain size does not monotonically increase withincreasing heat index.


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the top, middle, and bottom region of the weld nugget [89]. Such variation in grain size from bottom to

top of the weld nugget is believed to be associated with difference in temperature pro file and heatdissipation in the nugget zone. Because the bottom of workpieces is in contact with the backing plate,

the peak temperature is lower and the thermal cycle is shorter compared to the nugget top. The

combination of lower temperature and shorter excursion time at the nugget bottom effectively retards

the grain growth and results in smaller recrystallized grains. It is evident that with increasing plate

thickness, the temperature difference between bottom and top of the weld nugget increases, resulting

in increased difference in grain size.

4.1.3. Recrystallization mechanismsSeveral mechanisms have been proposed for dynamic recrystallization process in aluminum

alloys, such as discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystalliza-

tion (CDRX), and geometric dynamic recrystallization (GDRX) [97,101–106]. Aluminum and itsalloys normally do not undergo DDRX because of their high rate of recovery due to aluminum ’s highstacking-fault energy [101,105]. However, particle-simulated nucleation of DDRX is observed in

alloys with large (>0.6 mm) secondary phases [101–106]. The DDRX is characterized by nucleationof new grains at old high-angle boundaries and gross grain boundary migration [97]. On the other

hand, CDRX has been widely studied in commercial superplastic aluminum alloys [107–111] andtwo-phase stainless steels [112–114]. Several mechanisms of CDRX have been proposed wherebysubgrains rotate and achieve a high misorientation angle with little boundary migration. For example,


Fig. 22. Grain size distribution in various locations of 7050Al weld nugget [100].


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mechanisms include subgrain growth [107], lattice rotation associated with sliding [108,111], and

lattice rotation associated with slip [114].

As for dynamic nucleation process in the nugget zone of FSW aluminum alloys, CDRX [6,75,84],

DDRX [67,95,98], GDRX [69,115], and DRX in the adiabatic shear bands [116] have been proposed

to be possible mechanisms. Jata and Semiatin [6] were the first to propose CDRX as operative dynamicnucleation mechanism during FSW. They suggested that low-angle boundaries in the parent metal are

replaced by high-angle boundaries in the nugget zone by means of a continuous rotation of the original

low-angle boundaries during FSW. In their model, dislocation glide gives rise to a gradual relative

rotation of adjacent subgrains. Similarly, Heinz and Skrotzki [75] also proposed that CDRX is

operative during FSW/FSP. In this case, strain induces progressive rotation of subgrains with little

boundary migration. The subgrains rotation process gradually transforms the boundaries to high-angle

grain boundaries.

However, it is important to point out that many of the recrystallized grains in the nugget zone are

finer than the original subgrain size. Thus, it is unlikely that the recrystallized grains in the nugget zoneresult from the rotation of original elongated subgrains in the base metal. Recently, Su et al. [84]

conducted a detailed microstructural investigation of FSW 7050Al-T651. Based on microstructural

observations, they suggested that the dynamic recrystallization in the nugget zone can be considered a

CDRX on the basis of dynamic recovery. Subgrain growth associated with absorption of dislocation

into the boundaries is the CDRX mechanism. Repeated absorption of dislocations into subgrain

boundaries is the dominant mechanism for increasing the misorientation between adjacent subgrains

during the CDRX.

Alternatively, DDRX has been recently proposed as an operative mechanism for dynamic

nucleation process in FSW/FSP aluminum alloys based on recent experimental observations

[95,98]. Su et al. [95] reported generation of recrystallized grains of 0.1 mm in a FSP 7075Alby means of rapid cooling behind the tool. Similarly, Rhodes et al. [98] obtained recrystallized grains

of 25–100 nm in FSP 7050Al-T76 by using ‘‘plunge and extract’’ technique and rapid cooling. Theserecrystallized grains were significantly smaller than the pre-existing subgrains in the parent alloy, andidentified as non-equilibrium in nature, predominantly high-angled, relatively dislocation-free[95,98]. Su et al. [95] and Rhodes et al. [98] proposed that DDRX mechanism is responsible for

the nanostructure evolution.

The fact that recrystallized grains in the nugget zone of FSW/FSP aluminum alloys are

significantly smaller than the pre-existing subgrains in the parent alloy strongly suggests that DDRXis the operative mechanism for recrystallization during FSW/FSP of aluminum alloys.

4.1.4. Precipitate dissolution and coarsening

As presented in Section 3.2, FSW/FSP results in the temperature increase up to 400–550 8C

within the nugget zone due to friction between tool and workpieces and plastic deformation aroundrotating pin [4,5,41,60–63,67,68]. At such a high temperature precipitates in aluminum alloys cancoarsen or dissolve into aluminum matrix depending on alloy type and maximum temperature.

Liu et al. [5] investigated the microstructure of a friction stir welded 6061Al-T6. They reported

that the homogenously distributed precipitates are generally smaller in the workpiece than in the

nugget zone. However, there were far fewer large precipitates in the nugget zone than in the base

material. This implies the occurrence of both dissolution and coarsening of precipitates during FSW.

Recently, Sato et al. [61] examined the microstructural evolution of a 6063Al-T5 during FSW using

TEM. They did not observe precipitates within the nugget zone, indicating that all the precipitates

were dissolved into aluminum matrix during FSW. More recently, Heinz and Skrotzki [75] also

reported complete dissolution of the precipitates in FSW 6013Al-T6 and 6013Al-T4 with a tool



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rotation rate of 1400 rpm and a traverse speed of 400–450 mm/min. Similarly, in FSW 7XXXaluminum alloys (7075Al-T7451), Jata et al. [92] also observed the absence of strengthening

precipitates in the nugget zone, indicating complete dissolution of the precipitates. The overall

response includes a combination of dissolution, coarsening and reprecipitation of strengthening

precipitates during FSW/FSP.

4.1.5. Texture

Texture influences a variety of properties, including strength, ductility, formability and corrosionresistance. As mentioned earlier, the FSW material consists of distinct microstructural zones, i.e.,

nugget, TMAZ, HAZ and base material. Each zone has different thermo-mechanical history. What is

even more complicated for FSW is that the nugget region consists of sub-domains. For example, the

top layer undergoes deformation by shoulder after the pin has passed through. In addition, depending

on the tool rotation rate and traverse speed, the nugget region can contain ring pattern or other

microstructural variations. A few texture studies of FSW aluminum alloys have been reported [117–120]. In the last decade, the use of microtexture using orientation imaging microscopy (OIM) has

proved to be a very valuable tool in not only obtaining the texture information, but also establish the

grain boundary misorientation distribution data from same set of experiments.

Sato et al. [118] and Field et al. [119] have reported detailed texture analysis through the FSW

welds. The overall plots of grain boundary misorientation distribution showed that the nugget region

predominantly consisted of high-angle grain boundaries. However, the microtexture results showed

complex texture pattern. Sato et al. [118] noted that the Goss orientation in the parent 6063Al changed

to shear texture component with two types of orientation in the center of the nugget. The pole figureswere examined for the surface and center regions on both sides of the center line, i.e., on the advancing

and retreating sides. An important observation that emerged, by comparing pole figures at 2.5, 3.3, and4 mm away on both sides from the center, was that the weld center roughly contained {1 1 0}

h0 0 1

iand {1 1 4}h2 2 1i shear texture components. However, these components were rotated around the‘normal direction’, the direction of the axis of pin. Both these components were also observed by Fieldet al. [119], including the rotational aspect of the texture component from the advancing side to the

retreating side. During FSW, the material undergoes intense shearing and dynamic recrystallization

concurrently. One of the key issues to understand is how nucleation of new grains and continuous

deformation influence the final texture results. In addition, it is important to separate out the effect of

final deformation by shoulder through the forging action after the pin has passed. The deformationunder shoulder is likely to influence the final texture significantly. It adds a shear deformationcomponent at lower temperature to the recrystallized volume processed by the pin.

4.2. Thermo-mechanically affected zone

Unique to the FSW/FSP process is the creation of a transition zone—thermo-mechanicallyaffected zone (TMAZ) between the parent material and the nugget zone [4,15,41], as shown in Fig. 17.

The TMAZ experiences both temperature and deformation during FSW/FSP. A typical micrograph of

TMAZ is shown in Fig. 23. The TMAZ is characterized by a highly deformed structure. The parent

metal elongated grains were deformed in an upward flowing pattern around the nugget zone. Althoughthe TMAZ underwent plastic deformation, recrystallization did not occur in this zone due to

insuf ficient deformation strain. However, dissolution of some precipitates was observed in the TMAZ,as shown in Fig. 24c and d, due to high-temperature exposure during FSW/FSP [61,84]. The extent of

dissolution, of course, depends on the thermal cycle experienced by TMAZ. Furthermore, it was

revealed that the grains in the TMAZ usually contain a high density of sub-boundaries [61].



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4.3. Heat-affected zone

Beyond the TMAZ there is a heat-affected zone (HAZ). This zone experiences a thermal cycle,

but does not undergo any plastic deformation (Fig. 17). Mahoney et al. [61] defined the HAZ as a zoneexperiencing a temperature rise above 250 8C for a heat-treatable aluminum alloy. The HAZ retains


Fig. 23. Microstructure of thermo-mechanically affected zone in FSP 7075Al [15].

Fig. 24. Precipitate microstructures in the grain interior and along grain boundaries in: (a) base metal, (b) HAZ, (c) TMAZnear HAZ, and (d) TMAZ near nugget zone (FSW 7050Al-T651, tool rotation rate: 350 rpm, traverse speed: 15 mm/min)(after Su et al. [84]).


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the same grain structure as the parent material. However, the thermal exposure above 250 8C exerts a

significant effect on the precipitate structure.Recently, Jata et al. [92] investigated the effect of friction stir welding on microstructure of

7050Al-T7451 aluminum alloy. They reported that while FSW process has relatively little effect on

the size of the subgrains in the HAZ, it results in coarsening of the strengthening precipitates and the

precipitate-free zone (PFZ) increases by a factor of 5. Similar observation was also made by Su et al.

[84] in a detailed TEM examination on FSW 7050Al-T651 (Fig. 24b). The coarsening of precipitates

and widening of PFZs is evident. Similarly, Heinz and Skrotzki [75] also observed significantcoarsening of the precipitates in the HAZ of FSW 6013Al.

5. Properties

5.1. Residual stress

During fusion welding, complex thermal and mechanical stresses develop in the weld and

surrounding region due to the localized application of heat and accompanying constraint. Following

fusion welding, residual stresses commonly approach the yield strength of the base material. It is

generally believed that residual stresses are low in friction stir welds due to low temperature solid-state

process of FSW. However, compared to more compliant clamps used for fixing the parts in

conventional welding processes, the rigid clamping used in FSW exerts a much higher restraint

on the welded plates. These restraints impede the contraction of the weld nugget and heat-affected

zone during cooling in both longitudinal and transverse directions, thereby resulting in generation of

longitudinal and transverse stresses. The existence of high value of residual stress exerts a significant

effect on the postweld mechanical properties, particularly the fatigue properties. Therefore, it is of practical importance to investigate the residual stress distribution in the FSW welds.

James and Mahoney [93] measured residual stress in the FSW 7050Al-T7451, C458 Al–Li alloy,and 2219Al by means of X-ray diffraction sin2 c method. Typical results obtained in FSW 7050Al-

T7451 by pinhole X-ray beam (1 mm) are tabulated in Table 4. This investigation revealed following

findings. First, the residual stresses in all the FSW welds were quite low compared to those generatedduring fusion welding. Second, at the transition between the fully recrystallized and partially

recrystallized regions, the residual stress was higher than that observed in other regions of the weld.

Third, generally, longitudinal (parallel to welding direction) residual stresses were tensile and

transverse (normal to welding direction) residual stresses were compressive. The low residual stress


Table 4

Residual stress measurement (MPa) in FSW 7050Al-T6541 weld by pinhole beam X-ray (after James and Mahoney [93])

Location Distance from weld

centreline (mm)

Longitudinal Transverse

Retreating side Advancing side Retreating side Advancing side

Top surface 2 22 19 33 414 39 35 14 276 55 72 21 247 64 48 40 478 101 76 99 43

Root surface 1 13 42 28 52 123 36 52 48 54 71 195 61 30 55 55 103 48


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in the FSW welds was attributed to the lower heat input during FSW and recrystallization

accommodation of stresses [93].

Recently, Donne et al. [121] measured residual stress distribution on FSW 2024Al-T3 and

6013Al-T6 welds by using the cut compliance technique, X-ray diffraction, neutron diffraction and

high-energy synchrotron radiation. Six important observations can be made from their study. First, the

experimental results obtained by these measurement techniques were

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